In the field of biological signal monitoring, one is generally interested in signals that are closely related to the operation of the patient's heart. Such signals tend to have a number of common characteristics: (1) they tend to occur in the frequency range below 30 hertz; (2) they are generally at a very low level; (3) they are often associated with high levels of background noise; and (4) they are generally susceptible to the effects of motion-related artifacts.
In recent years, there has been a trend toward the development of biomedical monitoring equipment requiring acquisition and processing of multiple waveforms having the characteristics discussed above. Examples of systems requiring multiple waveforms include those for indirect measurement of blood pressure and those for indirect measurement of blood oxygen saturation. U.S. Pat. No. 4,269,193 issued to Eckerle and U.S. Pat. No. 4,423,738 issued to Newgard disclose noninvasive indirect measurement devices in which a plurality of pressure waveforms are processed to monitor blood pressure on a continuous basis. U.S. Pat. No. 4,407,290 issued to Wilber discloses an example of a system whereby light transmitted through an appendage at different wavelengths is used to determine the patient's arterial blood oxygen saturation.
In applications such as those discussed above, the biological signals of interest generally comprise a pulsatile waveform which is synchronous with the patient's heartbeat and which often includes harmonics of up to 20 hertz. The pulsatile component of the signal rides on a nonpulsatile component which may or may not be of interest, depending on the specific application. In situations where these signals are readily available, in continuous form, and at fairly high levels, they can be monitored using techniques which are known in the art. In some applications, however, these signals are only available in non-continuous form. For example, in the case of an optical oximeter, alternating pulse signals produced by light sources at different wavelengths are multiplexed onto a single line. In such systems, it is necessary to de-multiplex and reconstruct the signals. Most systems based on the acquisition of signals of this type employ commercially available sample-and-hold circuits which are followed by band-pass filters. These sample-and-hold circuits lack the extreme accuracy that is often required for acquisition of low-level biological signals. In addition, conventional sample-and-hold circuits tend to be sensitive to high frequency noise and often pass this noise into the system, thus corrupting the signal of interest.
The effectiveness of biological signal monitoring systems can be enhanced significantly by an improved detector which overcomes the problems discussed above. Specifically, there is a need for a detector circuit which is capable of acquiring very low level signals with a high degree of accuracy and which is relatively insensitive to high frequency noise signals.